The present invention relates to a technical field of a negative pressure booster which is used in a brake system or the like. More particularly, the invention relates to a technical field of a negative pressure booster which, in a low output region (low deceleration region), obtains a relatively low output (deceleration G) with a relatively small servo ratio and, in a high output region (high deceleration region), obtains a relatively high output (deceleration G) with a servo ratio larger than the servo ratio in the low output region, to thereby obtain a large output (deceleration G) with a small input (pedal depressing force) and to improve pedal feeling by shortening the pedal stroke.
So far, a brake system of an automobile such as a passenger car is using a negative pressure booster which utilizes a negative pressure as a brake booster. The above conventional generally used negative pressure booster is partitioned by a power piston into a constant pressure chamber into which a negative pressure is introduced by a power piston at all times and a variable pressure chamber in which the pressure varies. When the brake is normally applied by normally depressing the brake pedal, the input shaft moves forward causing a control valve to be changed over, and the atmosphere is introduced into the variable pressure chamber. Then, a pressure difference occurs between the variable pressure chamber and the constant pressure chamber, and the power piston moves forward. Therefore, the negative pressure booster boosts an input of the input shaft (i.e., depressing force of the pedal) at a predetermined servo ratio and outputs it. Due to the output of the negative pressure booster, a master cylinder generates a master cylinder pressure which works to operate wheel cylinders to effect the normal braking operation.
As a conventional negative pressure booster, International Laid-Open WO 2004/101340 is proposing a negative pressure booster which, in a low output region (low deceleration region, normal braking region) smaller than a predetermined output, boosts the input of the input shaft at a relatively small servo ratio to produce a relatively small output, and, in a high output region (high deceleration region) larger than the predetermined output, boosts the input of the input shaft at a servo ratio larger than the servo ratio in the low output region which is smaller than the predetermined output to produce a relatively large output and, further, shortens the stroke of the input shaft to produce a large output without increasing the stroke of the input shaft and, therefore, to obtain good operation feeling.
In the negative pressure booster disclosed in International Laid-Open WO 2004/101340, the vacuum valve is constituted as a moving vacuum valve having a moving vacuum valve seat on which the pressure in the variable pressure chamber of the negative pressure booster acts and on which a spring load, further, acts in a direction opposite to the direction in which the pressure in the variable pressure chamber acts. While the negative pressure booster is in operation, if the pressure in the variable pressure chamber of the negative pressure booster is in a low pressure region of lower than a predetermined pressure, the moving vacuum valve seat does not move since the force for pushing the moving vacuum valve seat based on the pressure in the variable pressure chamber is too small. Therefore, a balance position at which both the vacuum valve and the atmosphere valve close, does not shift relative to the valve body. Therefore, there is no change in the gap between a reaction disc which is reaction means and a valve plunger or the input shaft, and the negative pressure booster effects the boosting operation at a relatively small servo ratio.
Further, if the pressure in the variable pressure chamber of the negative pressure booster is in a high pressure region of higher than the predetermined pressure, the moving vacuum valve seat moves backward (toward the input side) relative to the valve body overcoming the spring load since the force for pushing the moving vacuum valve seat based on the pressure in the variable pressure chamber becomes great. Therefore, the balance position at which both the vacuum valve and the atmosphere valve close, shifts backward relative to the valve body. Therefore, the gap increases between a reaction disc and the valve plunger or the input shaft, and the jumping amount that varies depending upon an increase in the gap works to increase the output. The amount of backward motion of the moving vacuum valve seat increases with an increase in the pressure in the variable pressure chamber and, therefore, the balance position shifts backward relative to the valve body as the pressure in the variable pressure chamber increases. Therefore, the gap, too, increases with an increase in the pressure in the variable pressure chamber, the jumping amount which varies depending upon an increase in the gap works to increase the output accompanying an increase in the pressure in the variable pressure chamber, and the servo ratio becomes greater than that of when the moving vacuum valve seat does not move. According to the negative pressure booster disclosed in the patent document 1, as described above, the timing for changing the servo ratio from a small servo ratio in the low output region over to a large servo ratio in the high output region is controlled, i.e., the timing at which the moving vacuum valve seat starts moving is controlled depending upon the pressure in the variable pressure chamber of the negative pressure booster which is on the input side.
According to the negative pressure booster disclosed in the patent document 1, however, the pressure-receiving area of the moving vacuum valve seat must be increased when it is attempted to easily control the moving vacuum valve seat by reliably operating the moving vacuum valve seat with the pressure in the variable pressure chamber when the pressure in the variable pressure chamber has exceeded the predetermined pressure. If the pressure-receiving area of the moving vacuum valve seat is increased, however, the sealing portion of the moving vacuum valve increases resulting in an increase in the size of the negative pressure booster and, besides, it becomes difficult to produce the moving vacuum valve seat maintaining high precision. Therefore, it is not allowed to so much increase the pressure-receiving area of the moving vacuum valve seat making it relatively difficult to control the moving vacuum valve seat. Besides, since the moving vacuum valve seat is moved by the controlled input pressure, it becomes more difficult to control the timing for starting the motion of the moving vacuum valve seat.